In the field of materials science, elemental and structural analysis are widely carried out using an analytical electron microscope. When such an analysis is made with the microscope, a defocused electron beam of a relatively large diameter on the order of microns is first directed to a specimen to obtain a high-resolution transmission electron image at a high magnification, say of approximately one million. Then, the operator selects a microscopic region to be analyzed, such as a grain boundary, while observing the image. Subsequently, the beam is sharply focused to a diameter of the order of nanometers and is caused to irradiate the selected region. An electron diffraction pattern is then derived from this region. Also, x-rays produced from the irradiated region are detected by an x-ray detector. The output signal from the detector is fed to an energy-dispersive x-ray spectrometer to analyze the x-rays. This instrument is disclosed in an article entitled "Nanometer Probe Electron Diffraction in Correspondence with Image Field" by T. Tomita and Y. Harada, in Proc. XIth International Congress on Electron Microscopy, Kyoto, 1986, pp. 727-728.
This analytical electron microscope has two separate modes of operation. In particular, in one mode the defocused electron beam of the diameter on the order of microns is used to irradiate the specimen for observing an image. In the other mode the finely focused beam of the diameter on the order of nanometers is caused to irradiate on the specimen to analyze it. Therefore, it is difficult to accurately irradiate the electron beam on the desired microscopic region by observing the image. If the specimen is so moved that the desired region is placed on the optical axis, then it is possible to irradiate the electron beam on the desired region. However, since the electron beam and the specimen drift by the thermal fluctuation or the fluctuation of the external magnetic field etc., the positional relation between them changes. That is, if the focused electron beam irradiates on the specimen at a desired region at first, the irradiated region drifts with time.